It is known that high resolution technique for distributed fiber optic acoustic sensing rely on Coherent Rayleigh (CR) effect which is applied for measurements of dynamic strain induced in the sensing fiber by propagating high pressure acoustic waves along an infrastructure, such as, production wells, pipelines, or by generating acoustic high frequency “noise” in a structure that has a leak. To identify the source of acoustic signal, it is necessary to have a signal proportional to the dynamic strain, i.e., a signal having a linear response, and therefore, CR signal must be phase demodulated.
It is also known that the one of the most robust system architectures able to multiplex very large number of sensing elements is time divisional multiplexing (TDM) architecture. Combined with high sensitivity, high resolution and low cross-talk between sensing elements, TDM architecture offers so far a great performance for distributed sensing.
Conventional techniques related to the TDM approach in distributed sensing for low frequency sensing applications, such as, seismic, downwell acoustic, downwell seismic, streamer, etc., typically have acoustic bandwidth below a few kHz. Interrogation of the backscattered pulses is done by using heterodyne demodulation, where an array of sensors is interrogated with two optical pulses that are frequency shifted relative to each other and separated in time by a period set to twice the transit time in the sensor fiber.
However, TDM architecture has one fundamental shortcoming, i.e. bandwidth (BW) of the detection depends on the sensing length. In TDM approach, a transmitter launches a series of short laser pulses into a sensing fiber with repeatable sequences at a pulse repetition rate PRFL. As a result, data is interrogated, i.e. sampled at a sample rate fs equal to PRFL, and correspondingly, Nyquist frequency, i.e. BW of the detection is equal to half of the PRFL.
To improve the signal to noise ratio (SNR) in interrogating time-multiplexed interferometric sensors some conventional techniques use multiple interrogating pulse pairs and phase modulators as a frequency shifter, such that optical frequencies produced in different transmission time-slots are different. This approach allows an increased duty cycle, acceptable SNR and acoustic bandwidth of the detection, and reduces cross-talk during interrogation. Still, this approach is based on the heterodyne interrogation which limits detection of acoustic frequencies up to 20-30 kHz only.
To increase acoustic bandwidth to higher frequencies, it is necessary to increase the pulse interrogation rate over the length of sensing fiber, and use time sequences with multiple pulses to avoid heterodyne detection of interrogating pulses.
What is needed is a system for distributed sensing which provides simpler measurement techniques for acoustic signal, and which is configured to reject undesired contribution from backscattered signals.